We don't (didn't) have Voice, and our situation was a little different.  Dead batteries in the VRAD and no power at the CO, so even a generator would not have given us anything Uverse.

McDs was our line to the outside world, even after hours, with their free Wi-Fi.  And this was the first time we lost our landline service as well.  Cell service was maxed out, usually busy and dropped calls (not normal).

I'm sure others here can give you more ideas on maintaining the VoIP BBU during an extended outage.